At present, in the heavy oil cracking field, molecular sieves that can be used as cracking active components include Y molecular sieve, β molecular sieve, and ZSM molecular sieve, etc., wherein Y molecular sieve is used most widely. Existing methods for producing Y molecular sieve products in industrial production are essentially based on the method of using a crystallization directing agent (CDA) disclosed by GRACE Company (a US company) in the U.S. Pat. Nos. 3,639,099 and 4,166,099, and the ordinary Y molecular sieve products produced with such methods have crystal grains usually in about 1 μm grain size, with about 300-400 crystal cells in each dimension. In Y molecular sieve powder in ordinary grain size synthesized conventionally, the distribution percentage of pores in diameter smaller than 1 nm is 15-20%, the distribution percentage of pores in diameter within 1-10 nm range is 45-50%, and the distribution percentage of pores in diameter greater than 10 nm is 30-40%. For a macromolecular cracking reaction, the ideal pore diameter range suitable for raw material reaction and product diffusion is 1-10 nm. Though Y molecular sieve can be modified appropriately to an ideal pore diameter distribution range by post-modification, the final distribution of pore diameter range in the post-modified molecular sieve directly depends on the original pore diameter distribution in the molecular sieve; moreover, pore expansion has impacts on the skeleton structure of the molecular sieve, and thereby has impacts on the activity and stability of the molecular sieve.
In the prior art, the direct synthesis process refers to a process in which a Y molecular sieve (usually Na—Y molecular sieve) to be prepared is synthesized directly in one operation without post-treatment. At present, a CDA method is used conventionally. With that method, the chemical silica-alumina ratio (SiO2/Al2O3) in the synthesized Y molecular sieve is 3.5-5.5. To obtain a Y molecular sieve with a higher chemical silica-alumina ratio, expensive and highly toxic organic materials such as crown ether have to be added. In addition, in the preparation process of a Y molecular sieve, the lower the silica-alumina ratio is, the easier the preparation is; in contrast, the higher the silica-alumina ratio is, the harsher the conditions are, and the more difficult the preparation is. There are many influencing factors in preparation of a molecular sieve with a high silica-alumina ratio, such as the composition of the reaction mixture, the preparation method, the source of the reactants, the preparation of the directing agent, the acidity/alkalinity of the gel, and the conditions of crystallization, etc.
In CN103449468A, a Na—Y molecular sieve synthesis method is disclosed, comprising: mixing sodium silicate, sodium metaaluminate, and deionized water, and aging at 15-70° C. for 0.5-48.0 h to obtain a crystallization directing agent; mixing the crystallization directing agent, sodium silicate, an acidic aluminum salt, and sodium aluminate solution to a homogeneous state to prepare a silica-alumina gel; crystallizing the silica-alumina gel at 80-140° C. for 0.1-80.0h; adding peroxide into the crystallized silica-alumina gel at a mole ratio of peroxide to Al2O3 in the gel equal to 0.05-20, and then continuing the crystallization for 5-20h. With that method, no organic or inorganic template agent is added, no post-treatment or modification is required, and a Y molecular sieve with a high silica-alumina ratio can be prepared directly in a short time, and crystallinity of the obtained molecular sieve is equal to or higher than 80%, silica-alumina ratio not lower than 5.8, and average grain diameter within 200-300 nm range. Though that method can be used to synthesize a Y molecular sieve with high silica-alumina ratio, the preparation process is complex, the grain diameter of the obtained molecular sieve is too small, and a specific amount of peroxide has to be added into the gel. Hence, the conditions of molecular sieve synthesis are demanding.
In U.S. Pat. Nos. 3,671,191 and 3,639,099, a CDA method is used to synthesize a Y molecular sieve, wherein a directing agent is prepared first; then, a silica-alumina gel is prepared; next, the aged directing agent is added, and crystallization is carried out at a high temperature. In the method described above, an inorganic acid and an aluminum salt are used to decrease the alkalinity of the reaction system, and thereby improve the silica-alumina ratio of the resultant molecular sieve. However, only an ordinary Y molecular sieve can be prepared with that method, and a directing agent has to be synthesized first in the preparation process. In addition, the preparation process involves over many steps and high cost.
In CN101481120A, a method for preparation of a Y molecular sieve through a rapid crystallization process is disclosed. In that method, first, a silica-alumina gel is prepared from a silica source, an alumina source, and an alkali source; then, a W/O emulsion system is prepared from the silica-alumina gel, oil, surfactant, and co-surfactant; next, the W/O emulsion system is transferred into a reactor for rapid crystallization. The method employs an expansive surfactant to prepare the Y molecular sieve, and the preparation process is complex; consequently, the preparation cost is severely increased.
In CN1209358A, a Y zeolite rich in secondary pores is disclosed. Specifically, a method for preparation of a zeolite is disclosed, wherein Na—Y zeolite is used as the initial powder, and ammonium exchange is carried out first, to release Na+; then, hydrothermal treatment and acid extraction are carried out twice, wherein the second round of hydrothermal treatment and second round of acid extraction are carried out after the first round of hydrothermal treatment and first round of acid extraction. In the obtained Y zeolite, the pore volume of pores in diameter greater than 2 nm accounts for 40-66% of the total pore volume. In hydrocracking process, the transformation of macromolecular aromatics in the raw material is affected adversely, and the distribution and quality of the prepared catalyst product should be further improved.
Viewed from the aspect of application of molecular sieve products with a cracking function in industrial catalytic processes, the performance of molecular sieve products mainly depend on the two aspects: selective absorptivity and reactivity. The molecules of the reactants can diffuse into the pore canals of the molecular sieve and have specific catalyzed reactions only if the molecular size of the reactant is smaller than the pore size of the molecular sieve and the molecules can overcome the surface energy barrier of the crystals in the molecular sieve; here, the diffusivity of the absorbed molecules through the pores and cages of the crystals in the molecular sieve plays a decisive role. Hence, it is desirable to overcome the drawback of existing Y molecular sieve products in ideal pore diameter distribution and provide a Y molecular sieve with pore diameter distribution suitable for macromolecular cracking reactions.